1. Field
The present disclosure relates to techniques for communicating optical signals. More specifically, the present disclosure relates to a multi-chip module (MCM) that includes a device having an optical eigenmode normal to a surface of the device.
2. Related Art
Engineers have recently proposed using a multi-chip module (MCM) (which is sometimes referred to as a ‘macro-chip’) to integrate a collection of semi-conductor chips together. This MCM offers unprecedented: computational density, energy efficiency, bisection bandwidth and reduced message latencies. These characteristics are obtained by photonically interconnecting multiple silicon chips into a logically contiguous piece of silicon, thereby integrating: multi-core, multi-threaded processors, system-wide interconnects and dense memories.
As shown in FIG. 1, in one configuration of proposed MCM 100, hybrid chips, including island chips 110 (such as memory and/or one or more processor cores) and photonic bridge chips 112, are arranged in a two-dimensional, multi-tiered array. In this MCM, an upward-facing island chip (such as island chip 110-1) in the lower tier in MCM 100 is coupled to a downward facing bridge chip (such as bridge chip 112-1) in the upper tier. In particular, in the regions where these chips overlap (indicated by the fine dashed lines in FIG. 1), communication occurs via proximity communication of electromagnetically coupled signals (which is referred to as ‘electromagnetic proximity communication’). For example, the proximity communication may include: communication of capacitively coupled signals (electrical proximity communication') and/or communication of optical signals (such as ‘optical proximity communication’ or OPxC). Consequently, bridge chips 112 may include optical transmitter and receiver circuits, as well as capacitive-proximity-communication circuitry. In addition, bridge chips 112 may include one or more processor cores and/or memory.
Bridge chips 112 in MCM 100 may communicate with each other using optical links. In particular, there may be: optical waveguides 114 in the upper tier, optical waveguides 116 in the lower tier (for example, in a base or routing chip, which is obscured by island chips 110 and bridge chips 112 in FIG. 1), and waveguide-based silicon photonic devices, such as: modulators, receivers, wavelength-division-multiplexing multiplexers and wavelength-division-multiplexing de-multiplexers. Therefore, communication between bridge chips 112 may occur via OPxC with: optical waveguides 114, optical waveguides 116 and/or island chips 110.
In order to ensure reliable, low-power, low bit error rate off-chip communication, bridge chips 112 typically need to be positioned with a lateral accuracy that is a fraction of the optical mode size used in the OPxC. Furthermore, this chip-to-chip separation typically needs to be controlled to within a few microns to ensure the fidelity of the communication channels. Additionally, the chip alignment needs to be maintained while providing power to and removing heat from the components in MCM 100.
A variety of techniques have been proposed to implement OPxC between face-to-face chips, such as: grating-coupler based OPxC, reflecting-mirror based OPxC, and OPxC using ball lenses in etch pits. However, all these techniques usually require very accurate vertical and lateral alignment. Furthermore, while the etch-pit and ball-lens technique can potentially provide the accurate alignment needed for reliable OPxC, thermal expansion remains a concern. In addition, in order to accommodate the waveguide-based optical modulators and photo-detectors, bridge chips 112 are typically fabricated using silicon-on-insulator (SOI) process technology. However, this process technology is expensive, which increases the cost of bridge chips 112.
Hence, what is needed is an MCM without the above-described problems.